I'm building an app that can read text on websites, but I don't know how to select the text, do I need an extension for that or am I overlooking something? Could really use help to know what how to get this to work
Scraping other peoples' web sites is fraught with technical and moral hazard.
This sample shreds the MIT AI2 Build Server status page and shows its output as a bar graph.
So the reason for it to read text on websites is to use TextToSpeech option on it, its not like I'm trying to steal something from it. Would that still be wrong?
What you do depends on the data you intend to scrape. You might use an extension CustomWebView : An extended form of Web Viewer . The description indicates it can Find words and phrases in webview
Probably nothing wrong at all using your app to read text from a web page. It is OK to scrape a web page in general (unless the provider of that Web page specifically does not allow it). Said another way, don't worry about other's comments. It is usually OK to scrape a web page. 
Reading a web page for blind users would involve looking for accessibility-enabled markup like at
https://www.w3schools.com/html/html_accessibility.asp
If you know in advance which web pages you want, and if they are not loaded too heavily with JavaScript, you can do this in AI2.
Another sample app: